The function supplied as the :test argument to make-hash-table
specifies the `equivalence test' for the hash table it creates.
An object is `visibly modified' with regard to an equivalence test if there exists some set of objects (or potential objects) which are equivalent to the object before the modification but are no longer equivalent afterwards.
If an object O1 is used as a key in a hash table H and is then visibly modified with regard to the equivalence test of H, then the consequences are unspecified if O1, or any object O2 equivalent to O1 under the equivalence test (either before or after the modification), is used as a key in further operations on H. The consequences of using O1 as a key are unspecified even if O1 is visibly modified and then later modified again in such a way as to undo the visible modification.
Following are specifications of the modifications which are visible to the equivalence tests which must be supported by hash tables. The modifications are described in terms of modification of components, and are defined recursively. Visible modifications of components of the object are visible modifications of the object.
No standardized function is provided that is capable of visibly
modifying an object with regard to eq or eql.
As a consequence of the behavior for equal,
the rules for visible modification of objects not explicitly mentioned in this
section are inherited from those in Section 18.1.2.1 (Visible Modification of Objects with respect to EQ and EQL).
Any visible change to the car or the cdr of a cons
is considered a visible modification with regard to equal.
For a vector of type bit-vector or of type string, any visible change
to an active element of the vector,
or to the length of the vector (if it is actually adjustable
or has a fill pointer)
is considered a visible modification with regard to equal.
As a consequence of the behavior for equalp,
the rules for visible modification of objects not explicitly mentioned in this
section are inherited from those in Section 18.1.2.2 (Visible Modification of Objects with respect to EQUAL).
Any visible change to a slot of a structure
is considered a visible modification with regard to equalp.
In an array, any visible change
to an active element,
to the fill pointer (if the array can and does have one),
or to the dimensions (if the array is actually adjustable)
is considered a visible modification with regard to equalp.
In a hash table, any visible change
to the count of entries in the hash table,
to the keys,
or to the values associated with the keys
is considered a visible modification with regard to equalp.
Note that the visibility of modifications to the keys depends on the equivalence test
of the hash table, not on the specification of equalp.
Implementations that extend the language by providing additional mutator functions (or additional behavior for existing mutator functions) must document how the use of these extensions interacts with equivalence tests and hash table searches.
Implementations that extend the language by defining additional acceptable
equivalence tests for hash tables (allowing additional values for the :test
argument to make-hash-table) must document the visible components of these
tests.